Plastic packaging (e.g. plastic bags, plastic packaging of ham, cheese, yoghurt, other foods and consumer products, and plastic utensils) and other plastic waste (bottle crates, garden furniture, buckets, plastic sledges, car bumpers, petrol cans, pipes, spools, computer shells, TV shells, plastic parts of fridges, etc.) are the most problematic and the fastest-growing type of waste. According to common solutions, this type of waste is primarily landfilled, burned or used as filler. These solutions pollute the environment, are expensive and waste material that could be used as a raw material for new products.
Aside from that, everyone knows about plastic-waste recycling solutions where the waste is first sorted, then cleaned, and plastics of the same type are recycled into a uniform mass, granules or new products. The process of recycling is type-based, which means that, for example, LDPE (plastic packaging and bags), HDPE (plastic bags and thick-walled plastic products) or PET (plastic bottles of beverages) waste is washed, ground, dried and granulated. The plastics industry can use plastic granules made of one polymer as raw material for making new products. As sorting plastic waste by type is very expensive and time-consuming, mixed plastic waste that is not easy to sort is usually not recycled; it is either incinerated or landfilled.
There is no suitable solution for recycling polymers of different types. Compared to other materials, such as glass or metals, plastic polymers need to be processed longer to be recycled. The biggest problem is that polymers of different types do not mix because their molecular weight differs and they have long polymer chains. Heating polymers is not enough to break down their molecules. So, to be recycled, polymers need to be identical to achieve effective mixing. If different types of plastic are melted together, they do not mix, like oil and water—they form layers.
Such problems prevent the plastics industry from using unsorted mixed plastic waste, and sorting household and other plastic waste by types is very costly and almost impossible. The standard plastics industry that puts hundreds of millions of tonnes of plastic products on the market is set up to use ‘virgin’ or primary single-type plastic granules (LDPE, HDPE, PS, PP, PET, AB, composites (PS/PP, PP/PE, PS/PC) HIPS, EPS, PA, POM, PC, etc.), and its technological production solutions are not able to handle mixed or unclean plastic waste.
The mixing and melting of different types of plastic results in a very unstable mass that is difficult to handle in a common plastics manufacturing process in order to achieve a stable process and product. To mix different types of plastic waste in reprocessing, other materials are added to the mixture.
There are non-state-of-the-art solutions for making different products from recycled mixed plastic waste. Such solutions require other materials, chemical elements or pure, identifiable and cleaned plastic granules (virgin polymers) to be added to the recycled plastic mix in order to ensure better binding of the mixture. The materials used as fillers or additives are, for example, metal, wood, oil, talc, fly ash, potash, wood flour, or wood pulp. For example, a wood and polymer mixture is used to make WPC profiles, using the extrusion method. The weakness of such products is that the additives make these less weatherproof, and they are difficult to be nailed, milled or processed in any other way.
There are various state-of-the-art solutions for recycling mixed plastic waste into raw material for various products: for example, processing and compacting plastic waste by agglomerating, processing by extrusion or using the injection moulding technology. The extrusion process makes use of either single-screw extruders or multi-screw extruders (twin-, triple- or more screw extruders).
A common polymer extrusion process handles polymeric raw material that is exactly identified and has the very clear characteristics of a certain type of polymers. Such solutions do not allow for low-cost and fast recycling of mixed plastic waste, because the waste needs to be sorted, cleaned and dried beforehand, and the different types of plastic waste need to be processed separately.
The solution that is the most similar to the present invention is the manufacturing of products from raw material that has been recycled from mixed plastic waste, using a single-screw extruder. Examples of such a solution are patent applications US20050051646A1 (Collex Pty. Ltd.), Oct. 3, 2005 and US20020125600A1 (David Home), Dec. 9, 2002, that describe the recycling of mixed plastic waste to produce products, such as vineyard posts, oyster posts, pipe supports, slats, spat trays, manhole covers, and railroad ties, by moulding. The shortcoming of the said solution is that end products from recycled plastic waste cannot be made using a continuous process; instead, moulds need to be attached to the extruder. This makes production time-consuming, complex and costly.